The problem with trying to use the original switch board is that the MCU is on the switch board so the switch signal and aux LED signal didn’t need to be broken out at the junction from SW board to main board so there arnt pads for them. The stock driver only has 6 total pads between the two: it has 4 ground pads (to function as ground signal and mechanical stability), one signal pin and VCC.
That’s why i sort of gave up on the goal of compatibility even though my lower blard will physically interface perfectly with the stock upper board it’s actually a useless design feature. To use the stock switch board you’d have to solder air wires to the tiny MCU pads to pickup the switch and aux signals and while possible it’s an ugly solution.